I already got all outgoing calls to go out the LINE (fxo) port. I temporarily changed physical interfaces->phone port->outbound call route so that the part of its default value that had been ([1-9]x?*(Mpli)):pp} became
([1-9]x?*(Mli)):li}. Plus, I also, just to be safe, changed physical interfaces->phone port->primary line to be "PSTN". I made an outgoing call using my handset and it was confirmed that the caller id displayed on the called phone was that for my Verizon line on fxo. So, I was then sure all outgoing calls were going out on fxo.
I then once again tried sending a few faxes using my usb fax modem and still got the same failures: after only about 4 dtmf tones were dialed, I heard fast busy on the pc loudspeakers. So, unfortunately, using the Verizon line for outgoing calls did not seem to help.
Further, I have now also tried making an outgoing call using my laptop's builtin modem (as opposed to all the previous call attempts that had been made with a usb fax modem connected to my tower pc). I actually had some old software dial out to, believe it or not, a dialup ISP number. Same thing happened, a few digits dialed caused the fast busy signal.
At no time after any of these attempted calls did the OBI phone history log contain any indications that any of these calls were attempted.
If I have the time, I'll try taking my laptop and putting it right next to the Obi box and running a short phone line directly between the two and see if that helps. Maybe there is some sort of interaction with my house phone wiring or some of the other devices connected to my house phone wiring and if so, this will isolate that possible cause.